While in Delhi he started business as a moneylender. His cousins, Ranjit Singh and Gurbax Singh started Ranbaxy. Then Ranbaxy worked as the distributor for a Japanese pharmaceutical company, A. Shionogi, manufacturing vitamins and anti-TB drugs. When Ranbaxy failed to pay a loan, Bhai Mohan Singh bought the company for Rs. 2.5 lakh. He bought the company on 1st of August, 1952. He joined hands with Italian pharma company Lapetit Spa and later on bought this company too.
In the late 1960's with the launch of his super brand, Calmpose he made his mark in the pharmaceutical industry. In early 1970s when Indian adopted a system of process patents, Bhai Mohan Singh quickly realized the possibility of manufacturing any product in the world through reverse engineering. Thus, he established an R&D facility at Mohali.
By the year 1973, Ranbaxy Laboratories Ltd became a recognized brand of India. The same year, his eldest son Parviner Singh joined the company. In 1982, he became the Managing Director of Ranbaxy Laboratories Ltd. Bhai Mohan Singh had another son Analjeet Singh. He along with Analjeet laid the foundation of a new company Max.
A difference of opinion arose between Bhai Mohan Singh and Parvinder Singh regarding the functioning and expansion of Ranbaxy. In 1999 Bhai Mohan Singh was forced back and Parvider took over the company. This depressed Bhai Mohan Singh and he stopped participating in the company affairs.
Bhai Mohan Singh was the ex- vice president of the New Delhi Municipal Corporation (NDMC). For his contribution in civic matters he was awarded the Padma Shri. For his contribution to the industrial development of Punjab, the Punjab Goverment named an Industrial Township near Ropar after him. Padma Shri Bhai Mohan Singh died on March 27, 2006.